In the field of radioelectric transmissions, it is necessary to have antennas that on the one hand are lightweight and easy to use, or in other words essentially have little bulk, and on the other hand have high radioelectric efficiency, that is, in which the standing wave ratio is as low as possible in view of a high output.
Currently, high-output antennas with a low standing wave ratio are known, but only within a very narrow frequency band. To be able to cover a wide frequency band, it is known to cut the band into successive zones and to use a specific antenna for each zone in accordance with the communication frequency. It is also known to use only a single antenna, but to re-adapt it and/or to retune it electronically so that it has acceptable radioelectric characteristics at the selected communication frequency.
While antenna changing and/or retuning manipulations may be automated, such procedures are incompatible when using transmitter and receiver devices that change their operating frequency rapidly and frequently, in synchronism with one another. Such techniques are important for confidential communications. Nevertheless, the width of the frequency band in which these communication devices operate is implicitly limited to that within which the antenna can be used without disadvantages. However, the useful band width of an antenna has recently been extended, particularly in the very high frequency range, that is, between 30 and 90 MHz, for example.
In particular, one of the solutions adopted is described in U.S. Pat. No. 4,302,760 and has to do essentially with the construction of the antenna. Specifically, the antenna includes a plurality of linear conductor elements of different selected lengths that on the one hand are disposed parallel, electrically insulated from one another, at the top of a longitudinal support, and on the other hand have one of their ends electrically connected to a common line. According to the patentees of the aforementioned patent, an antenna of this type has given excellent results with a range of frequencies between 30 and 88 MHz. Unfortunately, it was not possible to exploit this solution successively in the field of high frequency communications, that is, below 30 MHz.
As a general rule, the length of a monopole antenna, that is, an antenna comprising a linear conductor element, corresponds to the square of the wavelength at the lowest frequency at which the antenna is used. To cover a frequency band above 3 MHz, one of the linear elements of the antenna in the aforementioned version must have a length of 18.2 m, which is entirely incompatible with mobile communications.
In the field of high frequencies, another version adopted by engineers is described in French Patent 2.597.266. This version suggests reducing the voltage amplitude ratio substantially for the lowest frequencies of the frequency band to be covered, by interposing electrically resistance means between the antenna and the transmission line to which it is connected. This solution does not yield good results except for a very narrow frequency band, that is, particularly when the ratio between the highest and the lowest frequency of the band is 3.
Moreover, if one seeks to apply this single solution to an antenna that operates over a wide band, for example, a band in which the ratio between the highest and the lowest frequency is 10, then a low voltage amplitude ratio can be obtained, but the output of the antenna will also be quite low.